Mechanical screening device for machine-harvested sugar cane

ABSTRACT

A mechanical screening device for loosening extraneous material from a mat of dirty machine-harvested sugar cane is made up of a series of heavy toothed rolls of large diameter which are arranged in parallel spaced relationshio on a rising incline. These toothed rolls are power-rotated to move the mat up the incline, each succeeding roll being driven at a speed about 15 percent faster than the roll next to and below it on the incline thus thinning out the mat as it travels up the incline. The spaces between the rolls are sufficiently large that most if not all of the cane stalks as well as extraneous material passes down between the rolls within the length of the mechanical screening device. The roll teeth are large with a straight leading edge having sufficient sweepback to prevent jamming by rocks but sufficiently steep to strike the mat decisively and impart a vigorous shaking to the cane.

United States Patent 1 Herkes 1 MECHANICAL SCREENING DEVICE FOR MACHINE-HARVESTED SUGAR CANE [76] Inventor: John w. Herkes, 425 High St.,

Santurce, PR. 96793 [22] Filed: Nov. 27, 1 972 21 Appl. No.: 309,896

[52] US. Cl 209/3, 209/233, 209/236, 209/146, 209/392, 198/211 [51] Int. Cl. B07b 1/14 [58] Field of Search 209/3, 104, 106,233,236, 209/235, 261, 146, 393, 392; 198/209, 211; 171/133; 130/276, 31

[56] References Cited UNITED STATES PATENTS 1,502,796 7/1924 Parker 209/233 1,864,484 6/1932 Currie 171/133 X 2,710,097 6/1955 Bolles 1 209/12 2,972,383 2/1961 Erdman 171/133 X 3,084,785 4/1963 Gransden 171/133 X 3,358,830 12/1967 Duncan 209/3 FOREIGN PATENTS OR APPLICATIONS 1,048,299 12/1953 France 171/133 1111 3,870,627 ]'Mar. 11,1975

Primary Examiner-Frank W. Lutter Assistant Examiner-Ralph J. Hill Attorney, Agent, or FirmWilliam B. Walter [57] ABSTRACT A mechanical screening device for loosening extraneous material from a mat of dirty machine-harvested sugar cane is made up of a series of heavy toothed rolls of large diameter which are arranged in parallel spaced relationshio on a rising incline. These toothed rolls are power-rotated to move the mat up the incline, each succeeding roll being driven at a speed about 15 percent faster than the roll next to and below it on the incline thus thinning out the that as it travels up the incline. The spaces between the rolls are suffi' ciently large that most if not all of the cane stalks as well as extraneous material passes down between the rolls within the length of the mechanical screening device. The roll teeth are large with a straight leading edge having sufficient sweepback to prevent jamming by rocks but sufficiently steep to strike the mat decisively and impart a vigorous shaking to the cane.

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MECHANICAL SCREENING DEVICE FOR MACHINE-HARVESTED SUGAR CANE BACKGROUND OF THE INVENTION Due to the rising cost of labor and transportation throughout the years sugar companies in many parts of the world have had to discontinue hand harvesting of cane in favor of harvesting by machine. In hand harvesting the canes are manually cut at ground level, the leaves and tops removed, and a bundle of clean cane thus manually carried to a receiving station consisting of a cart, flume, railcar or truck.

Because canes are tough they are not broken open except at the ends in hand cutting, are only dirty at points where the stalk may lie on the ground during its growth, or where stacked prior to bundling, and contains no rocks because of hand bundling. Hand cut cane is ideal for processing into sugar juices of acceptable purity and clarity because of its cleanliness.

Because hand out cane need not be washed and because it is not broken open except at each end, very little juice is lost even if transported to the factory in flumes.

On the other hand, machine harvested cane in some methods of growing, and certain types of harvesting contains heavy amounts of dirt, leaves, sand, and rock which become so intermingled with the cane that much effort is required to prevent it from being carried into the sugar extraction process.

Sugar growing and harvesting varies considerably from region to region. In Australia varieties of sugar cane have been selected and put into production, a principle characteristic of which is that it grows erect. Although this type of growth is not uniform throughout Australia, in fields of erect cane no cane is on the ground and the stalks in each row present a vertical wall. Machines developed to harvest erect Australian cane merely cut the cane off at ground level, draw it back into conveyors in such a manner that the cane is not bent or broken, and is not allowed to touch the ground again.

While erect Australian cane, and sugar cane in many other areas of the world is grown as a one year crop, usually on relatively level ground, in Hawaii a very lush growth of cane selected primarily for a high yield of sugar is grown as a two or three year crop. In its growth this lush cane becomes in this period an intertwined mass of canes which have grown up bent at all angles and lying on the ground. In contrast to a one year crop of about 40 tons of material to be harvested from each acre this lush two to three year crop amounts to an average of about 150 tons per acre at a density of approximately 13 pounds per cubic feet when piled.

Whereas a man can freely walk between rows spaced on 4'6 centers in Australian cane at harvest time an experienced machine operator in Hawaii sitting above the cane tops on his harvesting machine can only speculate on the location of rows of cane similarly spaced.

Ground conditions inwhich this cane is grown in Hawaii and harvested includes steep hills, deep, soft, wet soil, soil mixed with rock, rock with little evidence of soil, dry dusty soil and dry dusty soil mixed with rock.

Machines generally used for production harvesting of the Hawaiian cane are heavy push rakes mounted on the front of a heavy track-laying tractor which literally bulldoze the cane across the ground into mounds 6 to 12 feet in height, and in so doing pick up a great quantity of wet or dry soil, rocks, and leaves. This foreign material becomes attached and almost hopelessly entangled in the intertwined and interwoven stalks of cane, which average 12 feet in length.

Other types of harvesting, V-cutting and grab harvesting similarly result in much soil, rock, and leaves being intermixed with the harvested cane. Where adaptations of the machines used to harvest erect one year cane are used in Hawaii, using gathering devices to pick up recumbent cane, much less soil and rock as picked up. However, even this cane when harvested accumulates soil and leaves which must be removed before final processing. The cane left behind by each of these methods is swept up by much lighter tractors and rakes.

This final ground clearing operation of machine harvesting is responsible for the addition of a great proportion of dirt, sand, leaves, and rocks to the product. In many wet, rocky, or hilly areas trucks and trailers which will haul the cane to the factory cannot be operated off the road infield. Field cranes with heavy cane grabs are used to load hauling units from large piles on the ground, handling up to two tons of material at a time. Oftentimes such operations are repeated several times for the same material and each time more dirt is accumulated, canes are bent and broken open, and cane, rocks, leaves and dirt become more interwoven.

This cane is hauled in large semi-trailers of a unique design in which the 15 to 30 tons of material is suspended in a chain net. To unload the trailer one side of the net is raised so that the material is raised therein and rolled over the opposite side of a temporary storage area. This process can further entangle the cane and foreign material and break open stalks in highly stressed areas. A mill yard grab is next used to load the material, about 5 tons at a grab load, onto the factory conveyors.

This entire harvesting and hauling process has long been recognized as a brute force method of getting the cane from field growing conditions to harvested cane on factory conveyors and one which thoroughly entangles, dirties, and breaks open the cane. Any soil, sand, rocks and/or leaves remaining in the cane when fed into sugar extraction processes not only increases the cost of processing, but as the sugar juices are released from the cane stalk cells, the foreign material becomes saturated thereby and methods used to separate sugar juices from foreign material result in sugar losses into the discarded foreign material. These losses of sugar are extensive. In addition, repairs to factory equipment and time lost when the factory is not operating because of emergency repairs due to rocks is expensive.

A method commonly used to clean machine harvested cane as described above has been to pass the cane being conveyed into the factory through strong streams of water thus washing mud and dust away. Disadvantages of this so called wet cleaning process are the cost of the water and settling basins required to meet water pollution standards and the loss of sugar washed off the broken mangled cane.

For several years then, many attempts have been made to substitute for the wet cleaning processes a waterless or dry cleaning process using air blasts, peg tooth drums, and pocketed conveyors as shown in U.S. Pat. No. 3,384,233 to Bolles.

Although somewhat successful, these dry cleaners have yet to take the place of wet cleaners in any but experimental use in production sugar factories.

A major step in the cleaning of cane, whether by dry cleaning or wet cleaning processes is the separation of foreign materials from the cane by releasing them from the stalk surface and disentangling rocks, etc. from the blanket of material being conveyed into the sugar extraction processes.

Machinery designed for this purpose for other food products has included the shaker rolls used for cleaning of sugar beets as in Rienks US Pat. No. 1,899,292, and Armer US. Pat. No. 2,997,086 and in the cane sugar industry sunflower rolls and stooling drums. In each case the function of the device is to shake the foreign material free of the sugar beet or sugar cane.

FIGS. 4, 5, 6, and 7 of the Armer patent are illustrative of the theory of accelerating or tossing about of the sugar beet to shake loose the soil adhering thereto, while at the same time allowing the extraneous material to drop through the shaker rolls. Silver shows another variation in his leaf catcher for beet dumps, US. Pat. No. 1,895,268, in which beets, leaves and dirt cascade over an inclined toothed open belt moving in a direction opposed to that of the falling material onto a second inclined toothed open belt moving in the same direction as the beets.

Although it would be of assistance to the sugar cane industry to utilize the teachings of the prior art in the cleaning of sugar beets for the cleaning of sugar cane, such is not possible because of the great difference in the physical nature of the two products. Sugar cane stalks range in diameter at their base to about 1% inches tapering to leaves at the end of the stalk which may be up to feet in length. Sugar cane stalks in Hawaiian culture are seldom straight. Sugar cane stalks are strong, tough, and if not burned prior to harvest, have much leaf growth. If burned, the stalks will have less or possibly no leaves but will be more bent from the heat. Cane stalks can hairpin around moving parts of cane machinery, bunch up, and jam at the machinery unless especially designed to reduce or eliminate jamming.

As stated above, some sugar factories have either sunflower or stooling drums to assist in the cleaning process. In each case the drums are power-driven rolls, driven in the direction of travel of the material, and having spaced concentric plates attached thereto. The spaced plates of the sunflower drums have large gently rounded teeth in the form of a sine curve while the spaced plates of the stooling drums have large sharp teeth, the leading edges of which are radial. Because of the gentle form of the sunflower drum teeth very little loosening of extraneous material occurs as the dirty cane travels over them. The sharp teeth of the stooling drums are more aggressive and more effective in their cleaning. However, because of the radial leading edge, cane is damaged and rolls are jammed with rocks, stools, and cane to break drive components and thus requiring frequent shutdown of the entire plant for repair.

A common practice in sugar factories to remove large rocks from machine harvested sugar cane is to pass it through a trough of water called a rock gap. Sugar losses into this water are expensive and therefore an undesirable cost for removing rocks.

BRIEF SUMMARY OF THE INVENTION This invention is a mechanical screening device having a series of large, toothed, power-driven rollers in parallel spaced relationship on a steep incline. The toothed rollers are driven at a high speed in a direction as to propel material to be screened up the incline as it is also progressively passed down between the rollers. The roller teeth havean angle of sweepback sufficient to allow material stuck thereby to not jam against adjacent rollersand yet sufficiently erect to aggressively strike and raise the material being propelled by the rollers. The preferred sweepback angle is about 52 from radial.

The primary object of this invention is to break up a mat of machine-harvested sugar cane passing over and through it, thinning the mat and loosening extraneous adhered and intermingled material from the cane stalk so that other devices and apparatus can separate the extraneous material from the sugar cane thus cleaning the sugar cane.

Another object of this invention is to remove large rocks from the mat of cane without the use of water.

Another object of this invention is when used with other apparatus to clean mechanically harvested sugar cane using as little water as possible while functioning in a reliable manner without much damage to or breaking of sugar cane stalks.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings FIG. 1 is a schematic side elevation of one version of my mechanical screening device.

FIG. 2 is a schematic side elevation of another version of my device.

FIG. 3 is an enlarged plan view of a portion (three toothed rolls) of my device in a plane parallel to the inclined plane of my device.

FIG. 4 is an enlarged end view of two adjacent rolls in my device to illustrate the intermeshing of the notched rings or teeth.

FIG. 5 is an enlarged side view of another version of notched ring for use in my device.

FIG. 6 is a schematic side elevation of a version of my device in which a few rolls each on a series of inclines provide a sawtooth arrangement.

DETAILED DESCRIPTION OF THE INVENTION The screening device 1 as shown in FIGS. 1 and 2 is so located that harvested sugar cane 3 is deposited near the left end of it by conveyor 4. The rolls 2 and driving arrangements 5 are supported on a framework 6 so that the rolls are an angle of inclination of between 20 and 35 from the horizontal. Where the harvested cane 3 to be processed has been chopped into short lengths before being deposited on the shaker and rock removal apparatus 1 the angle would be about 20 as in FIG. 1. If the cane 2 has not been chopped and therefore mostly consisting of long stalks it can be elevated on the shaker device 1 at a steeper angle without excessive fall back and thus would be at an angle up to 32 as in FIG. 2. This steeper angle will assist in rock removal and also allow for more rolls or a decreased length and thus demand less factory area.

All rolls 2 are driven in the direction of travel of the cane 3 to the right and therefore clockwise as shown in FIGS. 1 and 2.

If the blanket of cane 3 is fed onto the shaker rolls 2 from the preceding conveyor 4 at a blanket velocity of 20 to 40 feet per minute, which would be normal and preferable, the first roll 2 or two would be driven at about 30 RPM or a peripheral speed such that the tips of the roller teeth 7 will be traveling at approximately 377 feet per minute or about ten to twenty times the blanket speed. Succeeding rolls 2 are then driven each at a speed of approximately 15 percent higher progressively so that the ninth roll will be rotating at a top speed of 1,155 feet per minute.

One way of increasing roller speed progressively is that shown here in FIG. 3 where the second roll 2 is chain driven by the first roll 2 with a 15 tooth driving sprocket 10 on the first roll and a 13 tooth driven sprocket 11 on the second roll.

In the arrangement illustrated here three 30 horsepower motors l2 are used to drive the entire assembly of shaker rolls 2.

The construction of the rolls 2 is as illustrated in FIGS. 3 and 4 simple and rugged with a series of 48 inch diameter notched rings 13 made of inch steel plate welded coaxially to the outside of a 30 inch schedule 30 pipe 14 which has a wall thickness of fiveeighths inches with the notched rings 13 spaced at 12 inches on center. Tooth tips of the notched rings are thus 9 inches above the roll surface. The ring spacing from the roll ends will differ by 6 inches in alternate rolls 2 so that when assembled the rings 13 intermesh leaving a generous 11% inches opening between rings 13 on each roll 2 with the rings 13 on the next roll centered on this space.

The 30 inch pipe 14 making up the cylindrical por tion of the roll 2 is about as long as the width of the screening device strengthened by 3 X 3 X /3 inch steel angles 15 welded to its inside surface and provided with 7 inch diameter solid steel stub axles 16 at each end. These axles 16 are suitably supported in bearings 17 at tached to the frame 6.

The ruggedness of this design attests to the heavy shock loads from rock, and the resistance of the tough sugar cane to the shaking and driving action.

The detail design of the teeth formed in the notched ring 13 is a principal element of my invention and highly responsible for the success of my sugar cane shaking and rock removal apparatus.

The leading edge 18 of each tooth is straight with a sweepback angle 19 of approximately 52 as shown. If the leading edge 18 were to be much steeper, as for instance as an extreme, the radial leading edge of the stooling drums as used in some sugar factories, rocks, the stool or root and base section of the sugar cane plant which is occasionally pulled out and mixed in with harvest cane, or heavy accumulations of cane stalks will be caught between the tooth of one roll and the teeth and cylindrical section of the next roll and jammed with such force as to break the driving chains, or other part of the system thus stopping the entire process feeding cane into the extraction process. The angle of sweepback N then of the tooth leading edge 18 must be great enough to prevent jamming by camming the material free and yet steep enough to vigorously move the cane 3 for effective loosening of extraneous material and to effectively move rocks.

I have found that a preferred sweepback angle if 52.

The tooth leading edge 18 in my shaker roll is not rounded at its terminal end 21. The tooth at its end 21 is about 2 inches long for adequate strength, and the trailing edge 22 is radial and straight. The space between teeth 7 is 2 inches on notched ring at the base and 10 inches at the tip.

In a preferred arrangement 30 inch diameter rolls are spaced at about 42 inches on center to leave a gap between rolls of 12 inches and a clearance between tooth tips on one roll and the adjacent roll of 3 inches. In some arrangements the roll spacing is not uniform throughout the length of my screening device and can have rolls closer for a smaller gap at one section and wider for a wider gap at another to control the type. size, and rate of flow of material passing between rolls at each section. Since the notched rings 13 are 12 inches on center on each roll 2 and alternately spaced on adjacent rolls 2 this leaves a clear space of 5% inches by 11 to 13 inches. This facilitates the passage of extraneous material and cane stalks down through these spaces between rolls.

In an alternate form of my invention the roller teeth 7 are bent as illustrated in FIG. 5. In this form, tooth 7C will be bent to the left, adjacent tooth 7D will be straight and the next tooth 7E will be bent to the right and so on. The purpose of this alternate form is to more effectively engage the short length of stalk in chopped cane and is a simple expedient obviating the need for closer spacing of notched rings 13. Preferably my device would consist of 9 to 15 rolls but as few as six or more than 15 could be used. Design limitations might require use of a series of short elements of a few rolls each in a sawtooth like arrangement.

MODE OF OPERATION OF INVENTION In operation of my mechanical screening device l a heavy tangled mat of machine harvested sugar cane stalks,-leaves and tops with adhering soil and sand entangled rocks and stools is deposited on the receiving end of the device 1 by a preceding conveyor 4 or chute. Preferably the preceding conveyor 4 has thinned the blanket down to a uniform height no greater than 12 inches and its velocity as it leaves this conveyor 4 from 20 to 40 feet per minute.

Since the material will drop onto my mechanical screening device 1 much, if not all, of its forward velocity will have been lost. Also as the material falls off the preceding conveyor 4 the blanket opens up. Thus as it drops onto the first rolls 2 the tooth action is very effective in engaging and shaking the material.

My device 1 moves the material, to the right as seen in FIGS. 1 and 2 and simultaneously elevates it at a blanket speed of about 40 to feet per minute. Great and vigorous action occurs within the blanket of material as it moves across and through my screening device 1 because rather than merely being carried or dragged by the conventional conveyor it is constantly struck by the teeth 7 of the notched rings 13. The action of the teeth 7 with sweptback leading edge 18 is not only to raise and let drop the pile locally, at a speed of six times per second to 18 times per second but to pull individual cane stalks through the pile so that rubbing action between stalks performs a self cleaning function. The spring of the cane stalks increases this action.

As the blanket of material moves up and across my screening device 1 individual cane stalks are constantly falling through the spaces between rollers 2 along with loosened soil and extraneous material. Large rocks released from the tangled cane stalks are either kicked up into a rock receiving conveyor or chute 23 in the apparatus with a low angle of inclination as shown in FIG. I or tumble down into a rock receiving chute or conveyor 24 on the apparatus having a steep angle of inclination as shown in FIG. 2. Actually it might be found desirable to have a rock receiver at each end of my screening device 1 as shown in FIG. 1.

All of the cane stalks fall through between the shaker rolls 2 so that none are kicked into the rock receiver 23 at the upper end.

Because the shaker rolls 2 are made with 30 inch diameter pipe 14 cane stalks will not wrap around the rolls 2 but are fed freely over and between them. This eliminates jamming of cane stalks along with the action of the sweptback teeth 7 which will not cause the cane stalks to wrap around the teeth 7 in the shape of a hairpin.

The overall action of my screening device 1 is (a) to take a heavy dense 12 inch thick mat of entangled dirty cane, rocks and leaves, (b) disentangle the cane, (c) loosen soil, dust, sand and leaves from the cane, (d) remove large rocks, and (e) spread out all this remaining material by increasing its speed so that subsequent apparatus can separate the cane stalks from the loose extraneous material.

I claim:

1. In a mechanical screening device comprised of a set of spaced parallel toothed cylindrical horizontal rolls arranged on a slope and power-rotated to pass material deposited thereon up the slope, with the teeth on each roll arranged in spaced rows and the rows of teeth on each roll centered on the space between rows of teeth of the adjacent rolls, the following:

a. each roller tooth has a substantially straight sweptback leading edge of about 52;

b. the rows of teeth are so located as to provide a lateral space between adjacent teeth of about 6 inches;

c. the roll spacing provides a space of about 12 inches between adjacent rolls;

d. the mechanical screening device also comprises means to drive the rolls at tooth tip speed of at least 350 feet per minute with each roll after the first one to three rolls rotated at a tooth tip speed which is about 15 percent higher than the next preceding roll; and

e. the effective length of the mechanical screening device is at least 21 feet;

whereby as a mat of dirty machine-harvested sugar cane is fed onto the lower end of the mechanical screening device, sugar cane stalks will be disentangled, extraneous material will be loosened from the cane stalks removing large rocks from the mat and all but rocks will progressively pass through the screening device, being spread out in length and thus separated farther because of the progressive increase in speed of the toothed rolls.

2. A screening device as claimed in claim 1 wherein said teeth are formed in notched rings which are concentric to the rolls and attached thereto in spaced relationship.

3. A screening device as claimed in claim I also comprising a rock receiver beyond the upper end of the screen.

4. A screening device as claimed in claim I also comprising a rock receiver at the bottom of the screening device.

5. A screening device as claimed in claim I wherein in each row of teeth every other tooth is in the plane of the row, one of the adjacent teeth in the row is bent so that the tooth tip is on one side of the row plane and the other tooth adjacent to the tooth in the row plane is bent so that the tooth tip is on the other side of the row plane so that each row of teeth acts on a greater width of material than .if all teeth were in the same plane.

6. A mechanical screening device as claimed in claim 1 in which the diameter of each roller is about 30 inches and the tooth tip diameter is about 48 inches.

7. A method of preparing a mat of tangled dirty machineharvested sugar cane for cleaning before extracting sugar comprising:

a. feeding the mat of tangled sugar cane off the end of a conveyor at a maximum mat thickness of about 12 inches onto the lower end of a mechanical screening device comprised of a set of spaced parallel horizontal toothed 'rolls with the teeth on each roll arranged in rows and the rows of teeth on each roll centered on the space between rows of teeth of the adjacent rolls through which material may pass;

b. rotating the rolls, to pass material thereon up a slope, at a tooth tip speed of about 380 feet per minute on the first one to three rolls, rotating each succeeding roll at a speed approximately 15 percent higher, thus disentangling the cane stalks and pulling the portion of the stalks at the bottom of the mat and advancing the stalks at a higher speed than those stalks not contacting the teeth;

c. the tooth rolls progressively raking the bottom of the mat, pulling individual stalks loose whereby cane stalks are spread out into a thinner mat of separated stalks and fed through between the rolls together with soil, sand, small rocks, and leaves from the cane stalks as the mat passes over the rolls; and

d. any rocks mixed in with the cane are thus struck loose from the cane and onto the top of the material to roll back down the slope of the mechanical screening device into a rock receiver or driven by the tooth rolls up into a rock receiver beyond the last roll. 

1. In a mechanical screening device comprised of a set of spaced parallel toothed cylindrical horizontal rolls arranged on a slope and power-rotated to pass material deposited thereon up the slope, with the teeth on each roll arranged in spaced rows and the rows of teeth on each roll centered on the space between rows of teeth of the adjacent rolls, the following: a. each roller tooth has a substantially straight sweptback leading edge of about 52*; b. the rows of teeth are so located as to provide a lateral space between adjacent teeth of about 6 inches; c. the roll spacing provides a space of about 12 inches between adjacent rolls; d. the mechanical screening device also comprises means to drive the rolls at tooth tip speed of at least 350 feet per minute with each roll after the first one to three rolls rotated at a tooth tip speed which is about 15 percent higher than the next preceding roll; and e. the effective length of the mechanical screening device is at least 21 feet; whereby as a mat of dirty machine-harvested sugar cane is fed onto the lower end of the mechanical screening device, sugar cane stalks will be disentangled, extraneous material will be loosened from the cane stalks removing large rocks from the mat and all but rocks will progressively pass through the screening device, being spread out in length and thus separated farther because of the progressive increase in speed of the toothed rolls.
 1. In a mechanical screening device comprised of a set of spaced parallel toothed cylindrical horizontal rolls arranged on a slope and power-rotated to pass material deposited thereon up the slope, with the teeth on each roll arranged in spaced rows and the rows of teeth on each roll centered on the space between rows of teeth of the adjacent rolls, the following: a. each roller tooth has a substantially straight sweptback leading edge of about 52*; b. the rows of teeth are so located as to provide a lateral space between adjacent teeth of about 6 inches; c. the roll spacing provides a space of about 12 inches between adjacent rolls; d. the mechanical screening device also comprises means to drive the rolls at tooth tip speed of at least 350 feet per minute with each roll after the first one to three rolls rotated at a tooth tip speed which is about 15 percent higher than the next preceding roll; and e. the effective length of the mechanical screening device is at least 21 feet; whereby as a mat of dirty machine-harvested sugar cane is fed onto the lower end of the mechanical screening device, sugar cane stalks will be disentangled, extraneous material will be loosened from the cane stalks removing large rocks from the mat and all but rocks will progressively pass through the screening device, being spread out in length and thus separated farther because of the progressive increase in speed of the toothed rolls.
 2. A screening device as claimed in claim 1 wherein said teeth are formed in notched rings which are concentric to the rolls and attached thereto in spaced relationship.
 3. A screening device as claimed in claim 1 also comprising a rock receiver beyond the upper end of the screen.
 4. A screening device as claimed in claim 1 also comprising a rock receiver at the bottom of the screening device.
 5. A screening device as claimed in claim 1 wherein in each row of teeth every other tooth is in the plane of the row, one of the adjacent teeth in the row is bent so that the tooth tip is on one side of the row plane and the other tooth adjacent to the tooth in the row plane is bent so that the tooth tip is on the other side of the row plane so that each row of teeth acts on a greater width of material than if all teeth were in the same plane.
 6. A mechanical screening device as claimed in claim 1 in which the diameter of each roller is about 30 inches and the tooth tip diameter is about 48 inches. 